As some of the posts have said, this particular lesson is confusing since the video displays entirely different code than the compass output renders. I understand that there’s different syntax depending on the version of MongoDB, but nonetheless that tripped me up. Next I think the variable name is confusing, atleast for me it was, because the const pipeline = []; is the variable name we’re using to nest the $lookup aggregation, which also has a pipeline field, that tripped me up as well because I was thinking the const pipeline = [] was supposed to be nested within the $lookup call. I understand that it’s probably just my lack of experience in this particular thing but I’m not the only one struggling with this particular lesson, so I just thought I’d give my feedback regarding this lesson. 
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same thing happened to me but I can also attribute this to lack of experience on this matter
I also found this a bit confusing and kind of overwhelming actually. After a bunch of hours testing, reading and after passing the test reading the detailed answer I’ve managed to understand the concept of the pipeline inside a $lookup inside a pipeline. Nice to see that I’m not the only one 